The invention relates to a movement damping apparatus, in which a magnetorheological fluid is forced through a flow path, having a device which produces a variable magnetic field and has pole surfaces in the flow path, via which pole surfaces the magnetic field acts on the magnetorheological fluid in order to control the flow characteristics.
In order to allow the flow characteristics of a magnetorheological fluid to be influenced by means of a magnetic field such that the flow resistance of the flow path is changed, various criteria must be satisfied. A magnetic field which is produced by a coil must be introduced into the magnetorheological fluid, for which purpose elements and parts of the apparatus which are provided for the direct path of the lines of force through the magnetorheological fluid should have higher magnetic permeability than other elements and parts which are not located on the direct path of the lines of force.
An apparatus which absorbs energy in the event of an impact is described, for example, in WO 2007/068436. The magnetic field is produced by a coil, which is provided with a C-shaped core composed of magnetically active material, in particular an iron core. An outlet channel with a rectangular cross section and which contains the flow path is passed through between the two pole surfaces of the C-shaped core, which are a short distance apart. The walls of the outlet channel, which rest on the pole surfaces, are composed of magnetically highly permeable material while, in contrast, the side walls are composed of a material which at least has a lower magnetic permeability than the magnetorheological fluid.
Movement or oscillation dampers with pistons and cylinders which contain a magnetorheological fluid and whose flow characteristics can be adjusted by means of a variable magnetic field are disclosed, for example, in DE 935831, DE 4433056 or U.S. Pat. No. 5,277,281. The magnetic field is produced by windings or coils whose axes extend in the movement direction of the piston, and therefore in the flow direction. The magnetorheological fluid is forced through a circumferential gap between the piston and the inner wall of the cylinder. DE 935831 discloses a stationary coil, which is arranged externally around the cylinder, while DE 4433056 contains the coil and a permanent magnet in the piston. According to U.S. Pat. No. 5,277,281, the piston is formed by a core around which the coil is wound, with the magnetic circuit including two annular areas of the cylinder wall, which are separated from one another. The lines of force therefore pass through the magnetorheological fluid with an axial separation, with the direction of the magnetic flux changing.
Since carbonyl-iron particles have no poles, the direction change is irrelevant, but the axial separation does play a role, since the particles must form new chains. Furthermore, the area or volume ratio of the coil to the core is very poor, leading to premature magnetic saturation of the core in the interior of the coil.